Smalltooth Cookiecutter Shark

The cookiecutter shark has an elongated, cigar-shaped body with a short, bulbously rounded snout. The nostrils have a very short flap of skin in front. The large, oval, green eyes are placed forward on the head, though not so that binocular vision is extensive. Behind the eyes are large spiracles, positioned on the upper surface of the head. The mouth is short, forming a nearly transverse line, and is surrounded by enlarged, fleshy, suctorial lips. The upper jaw has 30–37 teeth, and the lower jaw has 25–31, increasing with body size. The upper and lower teeth are extremely different; the upper teeth are small, narrow, and upright, tapering to a single, smooth-edged cusp. The lower teeth are also smooth-edged, but much larger, broader, and knife-like, with their bases interlocking to form a single saw-like cutting edge. The five pairs of gill slits are small. The pectoral fins are short and roughly trapezoidal in shape. Two spineless dorsal fins are placed far back on the body, the first originating just ahead of the pelvic fins and the second located just behind. The second dorsal fin is slightly larger than the first, and the pelvic fins are larger than either. The anal fin is absent. The caudal fin is broad, with the lower lobe almost as large as the upper, which has a prominent ventral notch. …

Inhabiting all of the world's major tropical and warm-temperate oceanic basins, the cookiecutter shark is most common between the latitudes of 20°N and 20°S, where the surface water temperature is 18–26 °C (64–79 °F). In the Atlantic, it has been reported off the Bahamas and southern Brazil in the west, Cape Verde, Guinea to Sierra Leone, southern Angola, and South Africa in the east, and Ascension Island in the south. In the Indo-Pacific region, it has been caught from Mauritius to New Guinea, Australia, and New Zealand, including Tasmania and Lord Howe Island, as well as off Japan. In the central and eastern Pacific, it occurs from Fiji north to the Hawaiian Islands, and east to the Galápagos, Easter, and Guadalupe Islands. Fresh wounds observed on marine mammals suggest this shark may range as far as California in warm years. Based on catch records, the cookiecutter shark appears to conduct a diel vertical migration up to 3 km (1.9 mi) each way. It spends the day at a depth of 1–3.7 km (0.62–2.30 mi), and at night it rises into the upper water column, usually remaining below 85 m (279 ft), but on rare occasions venturing to the surface. This species may be more tolerant of low dissolved oxygen levels than sharks in the related genera Euprotomicrus and Squaliolus. …

Best known for biting neat round chunks of tissue from marine mammals and large fish, the cookiecutter shark is considered a facultative ectoparasite, as it also wholly ingests smaller prey. It has a wide gape and a very strong bite, by virtue of heavily calcified cranial and labial cartilages. With small fins and weak muscles, this ambush predator spends much of its time hovering in the water column. Its liver, which can comprise some 35% of its weight, is rich in low-density lipids, which enables it to maintain neutral buoyancy. This species has higher skeletal density than Euprotomicrus or Squaliolus, and its body cavity and liver are proportionately much larger, with much higher oil content. Its large caudal fin allows it to make a quick burst of speed to catch larger, faster prey that come in range. The cookiecutter shark regularly replaces its teeth like other sharks, but sheds its lower teeth in entire rows rather than one at a time. A cookiecutter shark has been calculated to have shed 15 sets of lower teeth, totaling 435–465 teeth, from when it was 14 cm (5.5 in) long to when it reached 50 cm (20 in), a significant investment of resources. …